It is known to use an exhaust driven turbocharger or supercharger to improve the torque and emission performance of a diesel engine. For highly boosted diesel engines, both supercharged and turbocharged, the outlet temperature (Tcomp_out) of a compressor of such a booster is often the limiting factor in determining the maximum permitted boost level. There are a number of factors that determine the limit Tcomp_out, such as the material used to manufacture the intake manifold of the engine, but often the lowest of these factors is related to compressor coking caused by engine blow-by gases passing through the compressor at high temperatures. Coking progressively reduces compressor efficiency leading to higher intake temperatures, increased pumping losses and higher turbocharger speeds.
Therefore it is known that, in order to prevent such coking, the maximum permitted Tcomp_out and hence boost pressure and maximum power is restricted below a level that could be achieved and Tcomp_out is becoming the constraint on maximum power and torque deliverable by the engine.
For example, JP2005/180362 discloses a method and apparatus in which an electronic control unit performs a program including a step of detecting discharged air temperature TC, a step of judging whether or not the discharged air temperature TC is higher than a predetermined temperature TC(1) and a step of lowering the discharged air temperature TC by reducing supercharging pressure when the discharged air temperature TC is higher than the predetermined temperature TC(1). The predetermined temperature TC(1) is a temperature lower than a temperature at which coking occurs in the turbocharger.
It is a disadvantage of such a method of control that, for the majority of passenger car users, the proportion of time spent operating at conditions where the maximum permitted compressor outlet temperature is relevant is very small. This is because the maximum permitted compressor outlet temperature Tcomp_out_limit is typically only relevant when operating the engine close to the maximum rated power of the engine at warm ambient temperatures. Therefore for these users a fixed Tcomp_out_limit is an unnecessary restraint on the available performance of the engine.
The inventors have recognized the issues with the above approach and herein offer a method to at least partly address them. In one embodiment, a method for controlling operation of a compressor arranged to supply boosted air to an engine comprises varying a maximum permitted compressor outlet temperature based upon a function of compressor outlet temperature and operating time, and controlling the operation of the compressor so that the maximum permitted compressor outlet temperature is not exceeded.
In one example, the maximum permitted compressor outlet temperature may be based on the amount of time the compressor has been operated at various temperatures above a threshold. Thus, the temperature the compressor may operate at is not restricted solely on a preset maximum temperature but instead fluctuates throughout the life of the compressor, in order to maximize the time the compressor may spend at relatively high operating temperatures, thus maximizing boost pressure and increasing engine efficiency.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.